Semiconductor light emitting device and wafer

ABSTRACT

A semiconductor light emitting device includes a first layer made of at least one of n-type GaN and n-type AlGaN; a second layer made of Mg-containing p-type AlGaN; and a light emitting section provided between the first layer and the second layer. The light emitting section included a plurality of barrier layers made of Si-containing Al x Ga 1-x-y In y N (0≦x, 0≦y, x+y≦1), and a well layer provided between each pair of the plurality of barrier layers and made of GaInN or AlGaInN. The plurality of barrier layers have a nearest barrier layer and a far barrier layer. The nearest barrier layer is nearest to the second layer among the plurality of barrier layers. The nearest barrier layer includes a first portion and a second portion. The first portion is made of Si-containing Al x Ga 1-x-y In y N (0≦x, 0≦y, x+y≦1). The second portion is provided between the first portion and the second layer and is made of Al x Ga 1-x-y In y N (0≦x, 0≦y, x+y≦1). The Si concentration in the second portion is lower than a Si concentration in the first portion and lower than a Si concentration in the far barrier layer.

This is a division of application Ser. No. 12/505,053, filed Jul. 17,2009, now U.S. Pat. No. 8,039,830 which is incorporated herein byreference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2008-231097, filed on Sep. 9,2008; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a semiconductor light emitting device and awafer.

2. Background Art

A near-ultraviolet LED (light emitting diode) device (emissionwavelength being 400 nm or less) based on a nitride semiconductor isexpected to serve as a light source for use in a white LED, but has theproblem of low optical efficiency. As opposed to LD (laser diode), theemission efficiency of an LED device does not exceed the emissionefficiency of its light emitting section. To increase the emissionefficiency of the light emitting section, attempts have been made tofabricate a device based on a GaN substrate with low dislocationdensity. However, this is not a widely available technique because ofits high cost in addition to low efficiency.

Conventionally, it has been considered that the low emission efficiencyof an ultraviolet LED device is caused mainly by the large number ofcrystal defects such as dislocations in the light emitting layer. Inthis context, a growth technique is developed, in which a GaN layer isformed on a sapphire c-plane substrate via a buffer layer formed fromhigh Al composition AlGaN or AlN by high-temperature growth. Thistechnique can reduce the dislocation density to 10⁸-10⁹ m⁻³, which islower by 1/10 or less than conventional. However, near-ultraviolet LEDdevices based on this technique also have room for improvement.

Japanese Patent No. 2713094 discloses a technique for increasingemission efficiency using a double heterostructure in which an n-typegallium nitride-based compound semiconductor layer serves as a firstcladding layer, an In_(x)Ga_(1-x)N layer doped with a specific amount ofSi serves as a light emitting layer, and a p-type gallium nitride-basedcompound semiconductor layer doped with a specific amount of Mg servesas a second cladding layer.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided asemiconductor light emitting device including: a first layer made of atleast one of n-type GaN and n-type AlGaN; a second layer made ofMg-containing p-type AlGaN; and a light emitting section providedbetween the first layer and the second layer, the light emitting sectionincluding a plurality of barrier layers made of Si-containingAl_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), and a well layer providedbetween each pair of the plurality of barrier layers and made of GaInNor AlGaInN, the plurality of barrier layers having a nearest barrierlayer and a far barrier layer, the nearest barrier layer being nearestto the second layer among the plurality of barrier layers, the nearestbarrier layer including a first portion made of Si-containingAl_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), and a second portion providedbetween the first portion and the second layer and made ofAl_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), a Si concentration in thesecond portion being lower than a Si concentration in the first portionand lower than a Si concentration in the far barrier layer.

According to another aspect of the invention, there is provided asemiconductor light emitting device including: a first layer made of atleast one of n-type GaN and n-type AlGaN; a second layer made ofMg-containing p-type AlGaN; and a light emitting section providedbetween the first layer and the second layer and including a pluralityof barrier layers made of Si-containing Al_(x)Ga_(1-x-y)In_(y)N (0≦x,0≦y, x+y≦1), and a well layer provided between each pair of theplurality of barrier layers and made of GaInN or AlGaInN, each of thebarrier layers sandwiched between the well layers including a thirdportion provided on the first layer side and made of Si-containingAl_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), and a fourth portion providedon the second layer side and made of Al_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y,x+y≦1), a Si concentration in the fourth portion being higher than Siconcentration in the third portion.

According to another aspect of the invention, there is provided a waferincluding: a first layer made of at least one of n-type GaN and n-typeAlGaN; a second layer made of Mg-containing p-type AlGaN; and a lightemitting section provided between the first layer and the second layer,the light emitting section including a plurality of barrier layers madeof Si-containing Al_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), and a welllayer provided between each pair of the plurality of barrier layers andmade of GaInN or AlGaInN, the plurality of barrier layers having anearest barrier layer and a far barrier layer, the nearest barrier layerbeing nearest to the second layer among the plurality of barrier layers,the nearest barrier layer including a first portion made ofSi-containing Al_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), and a secondportion provided between the first portion and the second layer and madeof Al_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), a Si concentration in thesecond portion being lower than a Si concentration in the first portionand lower than a Si concentration in the far barrier layer.

According to another aspect of the invention, there is provided a waferincluding: a first layer made of at least one of n-type GaN and n-typeAlGaN; a second layer made of Mg-containing p-type AlGaN; and a lightemitting section provided between the first layer and the second layerand including a plurality of barrier layers made of Si-containingAl_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), and a well layer providedbetween each pair of the plurality of barrier layers and made of GaInNor AlGaInN, each of the barrier layers sandwiched between the welllayers including a third portion provided on the first layer side andmade of Si-containing Al_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), and afourth portion provided on the second layer side and made ofAl_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), a Si concentration in thefourth portion being higher than Si concentration in the third portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to afirst embodiment of the invention;

FIG. 2 is a graph illustrating the characteristics of the semiconductorlight emitting device according to the first embodiment of theinvention;

FIG. 3 is a graph illustrating the characteristics of the semiconductorlight emitting device according to the first embodiment of theinvention;

FIG. 4 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to asecond embodiment of the invention;

FIG. 5 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to athird embodiment of the invention;

FIG. 6 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to afourth embodiment of the invention;

FIG. 7 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to afifth embodiment of the invention;

FIG. 8 is a schematic cross-sectional view illustrating theconfiguration of a wafer according to a sixth embodiment of theinvention; and

FIG. 9 is a schematic cross-sectional view illustrating theconfiguration of a wafer according to a seventh embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will now be described with reference to thedrawings.

The drawings are schematic or conceptual. The relationship between thethickness and the width of each portion, and the size ratio between theportions, for instance, are not necessarily identical to those inreality. Furthermore, the same portion may be shown with differentdimensions or ratios depending on the figures.

In the present specification and the drawings, the same elements asthose described previously with reference to earlier figures are labeledwith like reference numerals, and the detailed description thereof isomitted as appropriate.

First Embodiment

FIG. 1 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to afirst embodiment of the invention.

As shown in FIG. 1, the semiconductor light emitting device 10 accordingto the first embodiment of the invention illustratively has a structureformed by stacking, on a substrate 110 having a sapphire c-planesurface, a first buffer layer 121 made of highly carbon-doped AlN, asecond buffer layer 122 made of high-purity AlN, a lattice-relaxed layer123 made of non-doped GaN, an n-type contact layer 130 made of Si-dopedn-type GaN, an n-type confinement layer (first layer) 131 made ofSi-doped n-type AlGaN, a light emitting section 140, a p-typeconfinement layer (second layer) 151 made of Mg-doped p-type AlGaN, anda p-type contact layer (third layer) 150 made of Mg-doped p-type GaN.

The p-type contact layer 150 is provided with a p-side electrode 160illustratively made of Au, and the n-type contact layer 130 is providedwith an n-side electrode 170 illustratively made of Ti/Pt/Au.

The n-type confinement layer 131 can be made of at least one of n-typeGaN and n-type AlGaN.

The light emitting section 140 has a multiple quantum well structure inwhich a barrier layer 141 made of Si-doped n-type AlGaInN and a welllayer 142 made of GaInN are alternately stacked six times.

Here, the aforementioned AlGaInN is Al_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y,x+y≦1).

The well layer 142 can be made of not only GaInN but also AlGaInN.

In other words, the light emitting section 140 includes a plurality ofbarrier layers 141 provided between the n-type confinement layer 131 andthe p-type confinement layer 151 and made of Si-containingAl_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), and a well layer 142 providedbetween each pair of the plurality of barrier layers 141 and made ofGaInN or AlGaInN.

Here, of the barrier layers 141, the one nearest the p-type confinementlayer 151 is referred to as “p-side barrier layer 141 a”. Furthermore,of the barrier layers 141, the one nearest the n-type confinement layer131 is referred to as “n-side barrier layer 141 b”. Furthermore, thebarrier layers 141 except the p-side barrier layer 141 a and the n-sidebarrier layer 141 b, that is, the barrier layers 141 sandwiched betweenthe well layers 142, are referred to as “inner barrier layers 141 c”. Inother words, the plurality of barrier layers 141 have a nearest barrierlayer and a far barrier layer. The nearest barrier layer is nearest tothe second layer among the plurality of barrier layers. In this case,the nearest barrier layer is the p-side barrier layer 141 a. The farbarrier layers include the n-side barrier layer 141 b and the innerbarrier layers 141 c.

In the semiconductor light emitting device 10 according to thisembodiment, the p-side barrier layer 141 a has a distributed Siconcentration. That is, the p-side barrier layer 141 a has a high Siconcentration on the well layer 142 side and a low Si concentration onthe p-type confinement layer 151 side.

More specifically, the p-side barrier layer 141 a includes a layeredfirst portion 143 a made of Si-containing Al_(x)Ga_(1-x-y)In_(y)N (0≦x,0≦y, x+y≦1) and a layered second portion 143 b provided between thefirst portion 143 a and the p-type confinement layer 151 and made ofAl_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1) which contains Si at aconcentration lower than the Si concentration in the first portion 143a.

Here, the Si concentration in the second portion 143 b is lower than theSi concentration in the barrier layers 141 except the p-side barrierlayer 141 a, that is, in the n-side barrier layer 141 b and the innerbarrier layers 141 c.

The Si concentration in the first portion 143 a can be not more than theSi concentration in the barrier layers 141 except the p-side barrierlayer 141 a, that is, in the n-side barrier layer 141 b and the innerbarrier layers 141 c. In other words, the Si concentration in the firstportion 143 a can be comparable to or lower than the Si concentration inthe n-side barrier layer 141 b and the inner barrier layers 141 c.

That is, in the semiconductor light emitting device 10 according to thisembodiment, the Si concentration is relatively high in the n-sidebarrier layer 141 b, the inner barrier layers 141 c, and the firstportion 143 a, and relatively low in the second portion 143 b.

In the following, an example configuration of the various layersdescribed above is presented. However, the invention is not limitedthereto, but can be variously modified.

The first buffer layer 121 can have, for instance, a carbonconcentration of 1×10¹⁹-5×10²⁰ cm⁻³ and a thickness of 3-20 nm.

The second buffer layer 122 can have, for instance, a carbonconcentration of 1×10¹⁶-1×10¹⁹ cm⁻³ and a thickness of approximately 2μm.

The lattice-relaxed layer 123 can have a thickness of e.g. 2 μm.

The n-type contact layer 130 can have, for instance, a Si concentrationof 1×10¹⁹-2×10¹⁹ cm⁻³ and a thickness of approximately 4 μm.

The n-type confinement layer 131 can be illustratively made of Si-dopedn-type Al_(0.13)Ga_(0.87)N with a Si concentration of 2×10¹⁸ cm⁻³ and athickness of 0.03 μm.

The p-type confinement layer 151 can be illustratively made of Mg-dopedp-type Al_(0.25)Ga_(0.75)N with a thickness of 24 nm.

Furthermore, in the p-type confinement layer 151, for instance, the Mgconcentration can be 3×10¹⁹ cm⁻³ on the second portion 143 b side, and1×10¹⁹ cm⁻³ on the surface side (on the opposite side from the secondportion 143 b, i.e., on the p-side electrode 160 side).

In the p-type contact layer 150, the Mg concentration can be 1×10¹⁹ cm⁻³on the p-type confinement layer 151 side, and 5-9×10¹⁹ cm⁻³ on thesurface side (on the opposite side from the p-type confinement layer151, i.e., on the p-side electrode 160 side).

The well layer 142 of the light emitting section 140 can beillustratively made of GaInN, and the peak wavelength of light emittedfrom the light emitting section 140 can be in the near-ultravioletregion, 370-400 nm (370 nanometers or more and 400 nanometers or less).

The n-side barrier layer 141 b and the inner barrier layer 141 c can beillustratively made of Si-doped n-type Al_(0.065)Ga_(0.93)In_(0.005)Nwith a Si concentration of 1.1-3.0×10¹⁹ cm⁻³ and a thickness ofapproximately 13.5 nm.

In the p-side barrier layer 141 a, the first portion 143 a on the welllayer 142 side can be made of Si-doped n-typeAl_(0.065)Ga_(0.93)In_(0.005)N with a Si concentration of 1.1-3.0×10¹⁹cm⁻³ like the n-side barrier layer 141 b and the inner barrier layer 141c, but with a thickness of approximately 4.5 nm.

On the other hand, the second portion 143 b with low Si concentrationcan be illustratively an Al_(0.065)Ga_(0.93)In_(0.005)N layer with a Siconcentration of 1×10¹⁶-3×10¹⁸ cm⁻³ and a thickness of approximately 4.5nm.

That is, the semiconductor light emitting device 10 includes an n-typeconfinement layer 131 made of at least one of n-type GaN and n-typeAlGaN; a p-type confinement layer 151 made of Mg-containing p-typeAlGaN; and a light emitting section 140 including a plurality of barrierlayers 141 provided between the n-type confinement layer 131 and thep-type confinement layer 151 and made of Si-containingAl_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), and a well layer 142 providedbetween each pair of the plurality of barrier layers 141 and made ofGaInN or AlGaInN.

In the light emitting section 140, the barrier layer 141 (p-side barrierlayer 141 a) nearest the p-type confinement layer 151 includes a firstportion 143 a made of Si-containing Al_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y,x+y≦1) and a second portion 143 b provided between the first portion 143a and the p-type confinement layer 151 and made ofAl_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1) which contains Si at aconcentration lower than the Si concentration in the first portion 143a.

In the semiconductor light emitting device 10 thus configured, in thebarrier layer (p-side barrier layer 141 a) nearest the p-typeconfinement layer 151, the second portion 143 b containing Si at a lowerconcentration than the rest is provided on the p-type confinement layer151 side. Thus, this embodiment can provide a semiconductor lightemitting device achieving near-ultraviolet emission with highefficiency.

In the following, the operation of the semiconductor light emittingdevice 10 according to this embodiment is described.

The heterojunction of a GaN mixed crystal has small discontinuity on thevalence band side, and the effect of confining holes is accordinglyweak. Hence, to improve the efficiency of the light emitting section140, it is considered important to strongly confine holes, which areminority carriers, in the well layer 142.

To enhance hole confinement using band curvature, the barrier layer 141was doped with Si at a high concentration of 1.1×10¹⁹ cm⁻³ or more. Itwas then found that the emission efficiency can be increased inphotoluminescence (PL). Here, by using a crystal with low dislocationdensity based on a high-temperature grown AlN buffer layer, high crystalquality can be maintained even if the barrier layer 141 is highly dopedwith Si.

However, in the case where the barrier layer 141 in an LED device washighly doped with Si as described above, sufficiently high efficiencywas not achieved.

This was presumably because doping with Si at high concentrationincreased the electron concentration and caused electrons to overflowinto the p-type semiconductor layer.

Thus, to prevent the overflow of electrons, attempts were made toincrease the thickness of the p-type confinement layer 151 formed on thelight emitting section 140 while increasing the Al composition ratio ofthe p-type confinement layer 151 to control energy in the p-typeconfinement layer 151.

Here, in an LED device based on a nitride semiconductor which emitsultraviolet or other short-wavelength light, the Mg concentration in thep-type confinement layer 151 is made higher than in a long-wavelengthLED device so that the operating voltage falls within a practical range.This is because a nitride semiconductor has a low hole mobility and alow carrier density, which result in increasing the operating voltageunless the Mg concentration of the p-type confinement layer 151 isincreased. That is, the piezoelectric field applied to the p-typeconfinement layer 151 hampers electric conduction and increases theoperating voltage. To cancel this piezoelectric field, a Mgconcentration of e.g. 1×10¹⁹ cm⁻³ or more is required.

Under the condition that the thickness of the p-type confinement layer151 was increased with its Al composition ratio increased, the Mgconcentration was increased to study the relationship between Mgconcentration and emission efficiency. However, also in this case, theemission efficiency was low.

To investigate the cause of the failure to increase the emissionefficiency, SIMS analysis was performed on the LED device. Then, in thep-type confinement layer 151, the Mg concentration exhibited a peculiardistribution being high on the light emitting section 140 side and lowon the surface side, and Mg was diffused into the light emitting section140. It was presumed that the emission efficiency was not increasedbecause of this diffusion of Mg into the light emitting section 140.

This analysis result suggested that the peculiar distribution of Mg wasascribable not only to heat, but also to other factors.

Through various experiments, the inventors have found that the Mgdistribution in the neighborhood of the interface between the p-typeconfinement layer 151 and the light emitting section 140 is stronglyaffected by the Si concentration of the barrier layer 141 (p-sidebarrier layer 141 a) in contact with the p-type confinement layer 151.More specifically, if the p-side barrier layer 141 a has a high Siconcentration, the electric field due to the resulting built-inpotential concentrates on the p-type confinement layer 151 and causes Mgatoms in the p-type confinement layer 151 to drift into the lightemitting section 140, thereby causing the anomalous diffusion of Mg.Thus, in the near-ultraviolet LED device, the diffusion of Mg with whichthe p-type confinement layer 151 is doped is significantly affected bythe aforementioned electric field in the neighborhood of the interfacebetween the p-type confinement layer 151 and the light emitting section140.

This invention has been accomplished on the basis of the above findings,and the semiconductor light emitting device 10 according to thisembodiment has a structure capable of controlling the aforementionedelectric field.

More specifically, the inventors have succeeded in increasing theemission efficiency by allowing the Si concentration on the p-typeconfinement layer 151 side of the p-side barrier layer 141 a to be lowerthan the Si concentration on the well layer 142 side. For instance, thesecond portion 143 b is not doped with Si, whereas the first portion 143a is doped with Si at high concentration.

The second portion 143 b with low Si concentration serves to avoid theaforementioned electric field which otherwise induces Mg diffusion. Thisprevents the anomalous diffusion of Mg into the light emitting section140, allowing the p-type confinement layer 151 to be highly doped withMg. Thus, the resistance of the p-type confinement layer 151 isdecreased, and consequently the emission efficiency can be increased.

By either increasing the thickness of the p-type confinement layer 151or increasing its Al composition ratio, the overflow of electrons can beavoided. This allows the barrier layer 141 to be highly doped with Si,and holes can be strongly confined in the well layer 142. Consequently,the emission efficiency can be further increased.

Here, the well layer 142 side of the p-side barrier layer 141 a ishighly doped with Si, which serves to strongly confine holes in the welllayer 142.

Furthermore, because the effect of the second portion 143 b with lowconcentration to prevent the anomalous diffusion of Mg allows the p-typeconfinement layer 151 to be highly doped with Mg, the operating voltagecan be decreased, which also results in the effect of improvingreliability.

More specifically, in the semiconductor light emitting device 10according to this embodiment, a second portion 143 b with low Siconcentration is provided on the p-type confinement layer 151 side ofthe p-side barrier layer 141 a. The electric field due to the built-inpotential caused by the high Si concentration of the n-typesemiconductor layer concentrates on the p-type confinement layer 151 andcauses Mg atoms to drift into the light emitting section 140, therebycausing the anomalous diffusion of Mg. The second portion 143 b servesto prevent this anomalous diffusion of Mg, and consequently the emissionefficiency can be increased. Furthermore, the resistance of the p-typeconfinement layer 151 can be decreased, which serves to decrease theoperating voltage and improve reliability.

Thus, the second portion 143 b is provided to decrease the electronconcentration in the vicinity of the interface with the p-typeconfinement layer 151 and prevent the electron overflow into the p-typeconfinement layer 151. Here, simultaneously, the hole concentration inthe vicinity of the interface also increases, and hence non-radiativerecombination at the interface also increases. However, this loss is lowand effectively irrelevant because of low dislocation density and use ofAlGaInN quaternary mixed crystal for the barrier layer 141.

Thus, this embodiment can provide a semiconductor light emitting device10 achieving near-ultraviolet emission with high efficiency.

The semiconductor light emitting device 10 according to this embodimentcan be fabricated as follows, for instance.

First, on a substrate 110 having a sapphire c-plane surface, metalorganic vapor phase growth is used to stack a highly carbon-doped AlNfilm (carbon concentration 1×10¹⁹-5×10²⁰ cm⁻³) with a thickness of 3-20nm serving as a first buffer layer 121, a high-purity AlN film with acarbon concentration of 1×10¹⁶-1×10¹⁹ cm⁻³ and a thickness of 2 μmserving as a second buffer layer 122, a non-doped GaN film with athickness of 2 μm serving as a lattice-relaxed layer 123, a Si-dopedn-type GaN film with a Si concentration of 1×10¹⁹-2×10¹⁹ cm⁻³ and athickness of 4 μm serving as an n-type contact layer, and a Si-dopedn-type Al_(0.13)Ga_(0.87)N film with a Si concentration of 2×10¹⁸ cm⁻³and a thickness of 0.02 μm serving as an n-type confinement layer 131.Then, a Si-doped n-type Al_(0.065)Ga_(0.93)In_(0.005)N film with a Siconcentration of 1.1-3×10¹⁹ cm⁻³ and a thickness of 13.5 nm serving as abarrier layer 141 (n-side barrier layer 141 b and inner barrier layer141 c) and a GaInN film with a thickness of 4.5 nm serving as a welllayer 142 are alternately stacked six times (except that a highlySi-doped n-type Al_(0.065)Ga_(0.93)In_(0.005)N film with a Siconcentration of 1.1-3.0×10¹⁹ cm⁻³ serving as a first portion 141 a hasa thickness of 4.5 nm), and an Al_(0.065)Ga_(0.93)In_(0.005)N film witha Si concentration of 1×10¹⁶-3×10¹⁸ cm⁻³ and a thickness of 4.5 nmserving as a second portion 143 b, a Mg-doped p-type Al_(0.25)Ga_(0.75)Nfilm with a thickness of 24 nm serving as a p-type confinement layer 151(the Mg concentration is 1.8×10¹⁹ cm⁻³ on the second portion 143 b side,and 1×10¹⁹ cm⁻³ on the surface side), and a Mg-doped p-type GaN filmserving as a p-type contact layer 150 (the Mg concentration is 1×10¹⁹cm⁻³ on the p-type confinement layer 151 side, and 5-9×10¹⁹ cm⁻³ on thesurface side) are sequentially stacked.

Next, these semiconductor layers are provided with electrodes by themethod illustrated below.

First, as shown in FIG. 1, in a partial region of these semiconductorlayers, using a mask, the p-type semiconductor layers and the lightemitting section 140 are removed by dry etching to expose the n-typecontact layer 130 to the surface. Then, entirely on the semiconductorlayers including the exposed n-type semiconductor layer, a SiO₂ film,not shown, is formed to a thickness of 400 nm using a thermal CVD(chemical vapor deposition) apparatus.

Then, to form a p-side electrode 160, first, a patterned resist forresist lift-off is formed on the semiconductor layer, and the SiO₂ filmon the p-type contact layer 150 is removed by ammonium fluoridetreatment. Then, on the region from which the SiO₂ film has beenremoved, a reflective conductive Ag film serving as a p-side electrode160 is formed to a thickness of 200 nm using a vacuum evaporationapparatus, for instance, and sintered for 1 minute in a nitrogenatmosphere at 350° C.

Then, to form an n-side electrode 170, a patterned resist for resistlift-off is formed on the semiconductor layer, and the SiO₂ film on theexposed n-type contact layer 130 is removed by ammonium fluoridetreatment. On the region from which the SiO₂ film has been removed, amultilayer film illustratively made of Ti/Pt/Au is formed to a thicknessof 500 nm to provide an n-side electrode 170.

Alternatively, it is also possible to use a high-reflectance silveralloy (e.g., containing Pd at approximately 1%). In this case, for goodohmic contact, the n-type contact layer is formed as a bilayerstructure, in which a highly-doped layer with a Si concentration of1.5×10¹⁹-3×10¹⁹ cm⁻³ is grown to approximately 0.3 μm as an electrodeformation portion. This can prevent reliability decrease due toprecipitation of Si.

Next, backside polishing, and cutting by cleavage or a diamond blade andthe like are performed. Thus, an individual LED device illustrativelyhaving a width of 400 μm and a thickness of 100 μm, that is, thesemiconductor light emitting device 10 according to this embodiment, isfabricated.

In the foregoing, the highly carbon-doped buffer layer (first bufferlayer 121) can be epitaxially grown by low-pressure metal organic vaporphase growth with a group V/group III raw material ratio from 0.7 to 50.

The other buffer layers (at least one of the second buffer layer 122 andthe lattice-relaxed layer 123) can be epitaxially grown by metal organicvapor phase growth at a higher temperature and a higher group V/groupIII raw material ratio than the epitaxial growth of the highlycarbon-doped buffer layer (first buffer layer 121).

The semiconductor light emitting device 10 according to this embodimentincludes at least an n-type semiconductor layer, a p-type semiconductorlayer, and a semiconductor layer including a light emitting section 140sandwiched between the n-type semiconductor layer and the p-typesemiconductor layer. The material of the semiconductor layers is notparticularly limited, but can be illustratively made of a galliumnitride-based compound semiconductor such as Al_(x)Ga_(1-x-y)In_(y)N(x≧0, y≧0, x+y≦1).

The method for forming these semiconductor layers is not particularlylimited, but can be based on such techniques as metal organic vaporphase growth and molecular beam epitaxial growth.

The material of the substrate 110 is not particularly limited, but thesubstrate 110 can be a substrate of sapphire, SiC, GaN, GaAs, Si and thelike. The substrate 110 may be finally removed.

In order to achieve highly efficient emission in the near-ultravioletrange by taking advantage of low-defect crystal, the semiconductor lightemitting device 10 according to this embodiment is based on varioustechniques to facilitate increasing the efficiency of the light emittingsection 140 itself and using the p-type confinement layer 151 having ahigh Al composition and a large thickness to prevent overflow ofelectrons from the light emitting section 140.

In the following, the high-temperature grown buffer layers (first andsecond buffer layer 121, 122, and lattice-relaxed layer 123), thebarrier layer 141, the well layer 142, and the p-type confinement layer151 in the semiconductor light emitting device 10 according to thisembodiment are described in detail.

The first buffer layer 121 made of highly carbon-doped AlN serves torelax the difference in crystal type from the substrate 110,particularly to decrease screw dislocations. The thickness of the firstbuffer layer 121 can be 3 nm or more and 20 nm or less.

The second buffer layer 122 made of high-purity AlN has a surfaceplanarized at the atomic level to serve to maximize the effect of thelattice-relaxed layer 123 made of GaN, which is grown thereon and servesfor defect reduction and strain relaxation. To this end, the thicknessof the second buffer layer 122 is preferably larger than 0.8 μm.Furthermore, to prevent warpage due to strain, the thickness ispreferably 4 μm or less.

That is, the buffer layers can include a second buffer layer 122provided above the substrate 110 and made of single crystal AlN, and afirst buffer layer 121 provided between the second buffer layer 122 andthe substrate 110, made of single crystal AlN, having a higher carbonconcentration than the second buffer layer 122, and having a thicknessof 3 nm or more and 20 nm or less.

While the second buffer layer 122 can be made of AlN, the invention isnot limited thereto. For instance, Al_(x)Ga_(1-x)N (0.8≦x≦1) can beused. In this case, the Al composition can be adjusted to compensate forthe wafer warpage.

The lattice-relaxed layer 123 serves for defect reduction and strainrelaxation by three-dimensional island growth on the second buffer layer122. To planarize the growth surface, the average thickness of thelattice-relaxed layer 123 needs to be 0.6 μm or more. From the viewpointof reproducibility and warpage reduction, the thickness of thelattice-relaxed layer 123 is preferably 0.8-2 μm.

By using these buffer layers, the dislocation density can be made 1/10or less of that of the conventional low-temperature grown buffer layer.This allows crystal growth at a high growth temperature and a high groupV/group III raw material ratio, which are normally difficult to use dueto anomalous growth. This avoids point defects, allowinghigh-concentration doping of AlGaN and the barrier layer 141 having ahigh Al composition.

The light emitting section 140 is formed by periodically andrepetitively stacking a barrier layer 141 made of Si-doped quaternarymixed crystal AlGaInN (Al composition being 6% or more and 8% or less,and In composition being 0.3% or more and 1.0% or less), and a welllayer 142 made of In_(0.05)Ga_(0.95)N.

The emission wavelength of the light emitting section 140 is 370 nm ormore and 400 nm or less. Because the absorption edge of GaN isapproximately 365 nm, the emission wavelength needs to be 370 nm ormore, where absorption by GaN is not high. To avoid absorption by GaNand increase emission efficiency, the emission wavelength is preferably380 nm or more and 385 nm or less. To form a potential deep enough toachieve highly efficient ultraviolet emission at wavelengths shorterthan 385 nm despite being longer than 375 nm, the Al compositionrequires 6% or more, but if it exceeds 9%, the crystal quality isdegraded. Slight In doping has the effect of improving crystal quality,and the effect manifests itself at an In composition of 0.3% or more.However, if the In composition exceeds 1.0%, the crystal quality isdegraded, decreasing the emission efficiency.

The n-side barrier layer 141 b, the inner barrier layer 141 c, and thefirst portion 143 a are highly doped with Si to increase electronconcentration in the well layer 142. This shortens the radiativerecombination lifetime and increases the efficiency. In this Si doping,its effect is insufficient at a concentration of 1.1×10¹⁹ cm⁻³ or less,and the crystal quality decreases at a concentration of 3.0×10¹⁹ cm⁻³ ormore.

Furthermore, because Si doping also serves to cancel the piezoelectricfield, a high Si concentration is required.

TABLE 1 illustrates the result of an experiment for studying therelationship between the Si concentration C3 of the barrier layer 141and PL intensity.

More specifically, this table illustrates the result of determining thedevice characteristics from the relationship between radiant flux and PLintensity for various Si concentrations C3 in the barrier layer 141(n-side barrier layer 141 b, inner barrier layers 141 c, and firstportion 143 a). In this table, the cross mark represents that theradiant flux is 10 mW or less, the triangle mark represents that theradiant flux is 10 mW or more and less than 13 mW, the circle markrepresents that the radiant flux is 13 mW or more and less than 15 mW,and the double-circle mark represents that the radiant flux is 15 mW ormore.

TABLE 1 C3 (10¹⁹ cm⁻³) 0.69-0.83 0.9 1.1 1.2 1.2 1.3 1.46 1.5 1.61 1.651.94 2.4 RESULT x xΔ Δ(x) Δ◯ Δ Δ Δ◯ Δ⊚ ⊚Δ ⊚ ◯ ◯

As shown in TABLE 1, as the Si concentration C3 increased, the PLintensity increased. The PL intensity was high at a Si concentration C3of 1.5×10¹⁹-1.66×10¹⁹ cm⁻³, and decreased at 1.94×10¹⁹ cm⁻³ or more.

It is considered that if the Si concentration C3 is too high, thecrystal quality is degraded, decreasing the efficiency. As the result ofstudying the relationship between Si concentration C3 and devicecharacteristics, the radiant flux was maximized at a Si concentration C3of 1.5×10¹⁹-1.65×10¹⁹ cm⁻³.

To increase the PL intensity, the Si concentration needs to be 1.1×10¹⁹cm⁻³ or more. Although the PL intensity is high even at a Siconcentration of 2.4×10¹⁹ cm⁻³, the number of pits slightly increases.Hence, in view of the lifetime of the LED device, the Si concentrationis preferably 2.4×10¹⁹ cm⁻³ or less.

A Si-doped GaN layer having a Si concentration of 3.0×10¹⁹ cm⁻³ wasformed. Then, its surface was slightly rough. Hence, it is consideredthat if the Si concentration is higher than this, the crystal quality issignificantly degraded. Thus, the Si doping concentration in the barrierlayer 141 (n-side barrier layer 141 b, inner barrier layers 141 c, andfirst portion 143 a) is preferably 3.0×10¹⁹ cm⁻³ or more.

In this example, the Si concentration in the first portion 143 a isequal to the Si concentration in the n-side barrier layer 141 b and theinner barrier layers 141 c. However, the invention is not limitedthereto, but the Si concentration in the first portion 143 a may bedifferent from the Si concentration in the n-side barrier layer 141 band the inner barrier layers 141 c. For instance, if the carrierconcentration in the well layer 142 is sufficiently high, the Siconcentration in the first portion 143 a may be lower than the Siconcentration in the n-side barrier layer 141 b and the inner barrierlayers 141 c. Also in this case, the Si concentration in the firstportion 143 a is set higher than the Si concentration in the secondportion 143 b.

From these results, the Si concentration is preferably 1.1×10¹⁹ cm⁻³ ormore and 3.0×10¹⁹ cm⁻³ or less.

That is, in the barrier layers (n-side barrier layer 141 b and innerbarrier layers 141 c) except the barrier layer 141 nearest the p-typeconfinement layer 151, the Si concentration is preferably 1.1×10¹⁹ cm⁻³or more and 3.0×10¹⁹ cm⁻³ or less. Furthermore, the Si concentration inthe first portion 143 a is preferably 1.1×10¹⁹ cm⁻³ or more and 3.0×10¹⁹cm⁻³ or less.

In the following, the first portion 143 a and the second portion 143 bare described.

In the semiconductor light emitting device 10 according to thisembodiment described above, the Si concentration varies stepwise, ordiscontinuously, between the first portion 143 a and the second portion143 b. In such cases, the first portion 143 a and the second portion 143b are clearly distinguished from each other.

However, the invention is not limited thereto. For instance, it is onlynecessary for the p-side barrier layer 141 a to contain Si at a lowerconcentration on the p-type confinement layer 151 side than on the welllayer 142 side. That is, the Si concentration may continuously decreasein the p-side barrier layer 141 a from the well layer 142 side towardthe p-type confinement layer 151 without clear distinction between thefirst portion 143 a and the second portion 143 b. Here, the portion witha relatively high Si concentration is regarded as the first portion 143a, and the portion with a relatively low Si concentration is regarded asthe second portion 143 b.

Alternatively, it is also possible to regard the first portion 143 a asthe p-side barrier layer 141 a and regard the second portion 143 b asanother barrier layer provided between the p-side barrier layer 141 aand the p-type confinement layer 151. However, it is assumed that thesecond portion 143 b is included in the p-side barrier layer 141 a.

With regard to the first portion 143 a and the second portion 143 b, inthe description of the above example, the Si concentration variesdiscontinuously in two steps. However, the invention is not limitedthereto, but the Si concentration may vary stepwise in three or moresteps, or vary continuously.

Also in the case where the Si concentration varies stepwise in three ormore steps, the portion with a relatively high Si concentration can beregarded as the first portion 143 a, and the portion with a relativelylow Si concentration can be regarded as the second portion 143 b.

In the following, for simplicity of description, it is assumed that theSi concentration varies discontinuously in two steps.

The thickness of the first portion 143 a may be equal to or differentfrom the thickness of the n-side barrier layer 141 b and the innerbarrier layer 141 c.

The thickness of the second portion 143 b can be set independent of thethickness of the first portion 143 a, the n-side barrier layer 141 b,and the inner barrier layer 141 c.

The thickness of the first portion 143 a and the second portion 143 b isdescribed below.

FIG. 2 is a graph illustrating the characteristics of the semiconductorlight emitting device according to the first embodiment of theinvention.

More specifically, this figure illustrates the result of an experimentfor studying the emission efficiency with the thickness of the firstportion 143 a and the thickness of the second portion 143 b each varied.The horizontal axis represents the total thickness D of the firstportion 143 a and the second portion 143 b, and the vertical axisrepresents radiant flux.

As shown in FIG. 2, when the total thickness D of the first portion 143a and the second portion 143 b was 9 nm, the radiant flux was maximized.When the total thickness D exceeded 9 nm, the emission efficiencydecreased. This is presumably because holes are trapped by the interfacebetween the second portion 143 b and the p-type confinement layer 151,increasing non-radiative recombination. In the case where the totalthickness D is smaller than 4 nm, diffusion of Mg cannot be prevented.

In this experiment, the thickness of the inner barrier layer 141 c ofthe light emitting section 140 is 13.5 nm. Hence, the total thickness Dof the first portion 143 a and the second portion 143 b is preferablysmaller than the thickness of the inner barrier layer 141 c of the lightemitting section 140.

Furthermore, in view of device reproducibility and productivity, thetotal thickness D of the first portion 143 a and the second portion 143b is preferably 7 nm or more and 12 nm or less.

Next, the thickness tb of the second portion 143 b is described.

TABLE 2 illustrates the result of an experiment for studying therelationship between the thickness tb of the second portion 143 b anddevice characteristics. More specifically, this table illustrates therelationship between the thickness tb of the second portion 143 b andaverage radiant flux. In this table, the cross mark represents that theradiant flux is 10 mW or less, the triangle mark represents that theradiant flux is 10 mW or more and less than 13 mW, the circle markrepresents that the radiant flux is 13 mW or more and less than 15 mW,and the double-circle mark represents that the radiant flux is 15 mW ormore.

TABLE 2 tb(nm) 0 4.5 9 RESULT X ◯ ⊚ Δ

As shown in TABLE 2, the radiant flux was maximized when the thicknesstb of the second portion 143 b was 4.5 nm, and decreased when it was 9nm. In the case where the thickness tb of the second portion 143 b was 0nm, that is, in the case of a comparative example including no secondportion 143 b, the radiant flux was low.

Furthermore, although not described in this table, the PL intensitysignificantly decreased when the thickness tb was 10 nm or more.

Hence, the thickness tb of the second portion 143 b is preferably 3 nmor more and 9 nm or less.

Furthermore, the reproducibility and productivity of devicecharacteristics are superior when the thickness tb of the second portion143 b is 3 nm or more and 6 nm or less.

In the semiconductor light emitting device 10 according to the firstembodiment, the thickness tb of the second portion 143 b is 4.5 nm, andthe thickness of the first portion 143 a is also 4.5 nm. In this case,the total thickness D of the first portion 143 a and the second portion143 b is 9 nm, which corresponds to the condition for maximizing theradiant flux described with reference to FIG. 2.

Next, the Si concentration of the second portion 143 b is described.

FIG. 3 is a graph illustrating the characteristics of the semiconductorlight emitting device according to the first embodiment of theinvention.

More specifically, this figure illustrates the result of an experimentfor studying the relationship between the Si concentration of the secondportion 143 b and emission efficiency. The horizontal axis representsthe Si concentration C3 of the second portion 143 b, and the verticalaxis represents average radiant flux.

As shown in FIG. 3, in the semiconductor light emitting device 10according to this embodiment, the average radiant flux was increasedwith the decrease of the Si concentration C3 of the second portion 143b, and maximized in the non-doped case (a Si concentration ofapproximately 1×10¹⁶ cm⁻³). This value of Si concentration, 1×10¹⁶ cm⁻³,is the concentration of Si contained even in the case withoutintentional doping, that is, the concentration at the background level.

From FIG. 3, the Si concentration C3 of the second portion 143 b can beset to 1×10¹⁶ cm⁻³ or more and 9.0×10¹⁸ cm⁻³ or less. Thereproducibility and productivity of device characteristics are superiorwhen the Si concentration C3 of the second portion 143 b falls withinthis range.

In the following, the Mg concentration of the p-type confinement layer151 is described.

In the semiconductor light emitting device 10 according to thisembodiment, the crystal quality of the light emitting section 140 isimproved by the barrier layer 141 made of quaternary mixed crystalAlGaInN. Hence, the p-type confinement layer 151 can be doped with Mg ata higher concentration than conventional.

TABLE 3 illustrates the result of an experiment for studying therelationship between the average Mg concentration C4 of the p-typeconfinement layer 151 and device characteristics. More specifically,this table illustrates the result of determining the devicecharacteristics from the relationship between radiant flux and PLintensity with respect to the average Mg concentration C4 of the p-typeconfinement layer 151. In this table, the cross mark represents that theradiant flux is 10 mW or less, the triangle mark represents that theradiant flux is 10 mW or more and less than 13 mW, the circle markrepresents that the radiant flux is 13 mW or more and less than 15 mW,and the double-circle mark represents that the radiant flux is 15 mW ormore.

TABLE 3 C4 (10¹⁹ cm⁻³) 0.7 0.9 1 1.1 1.15 1.21 1.3 1.6 1.75 1.94 RESULTX X Δ ◯ ◯ ◯ ⊚ Δ X Δ X Δ X X

As shown in TABLE 3, the radiant flux was increased with the increase ofthe average Mg concentration C4 of the p-type confinement layer 151, andmaximized when the average Mg concentration C4 was 1.15×10¹⁹ cm⁻³. Theradiant flux was decreased when the average Mg concentration C4 wasfurther increased.

From TABLE 3, the average Mg concentration C4 of the p-type confinementlayer 151 is preferably 0.9×10¹⁹-1.6×10¹⁹ cm⁻³.

However, this is the value of the average Mg concentration C4. Inpractice, the p-type confinement layer 151 has a distributed Mgconcentration.

That is, the Mg concentration of the p-type confinement layer 151 ishigh, 2×10¹⁹ cm⁻³, on the second portion 143 b side, and low, 8×10¹⁸cm⁻³, on the surface side (opposite side from the second portion 143 b).A good result was achieved in this case.

Thus, the Mg concentration of the p-type confinement layer 151 can beset to 8×10¹⁸-2×10¹⁹ cm⁻³. Furthermore, the average Mg concentration C4in the p-type confinement layer 151 can also be set to 2×10¹⁹ cm⁻³.

The p-type contact layer 150 also has a distributed Mg concentration.The efficiency was increased when the Mg concentration of the p-typecontact layer 150 was lower on the p-type confinement layer 151 sidethan on the surface side (opposite side from the p-type confinementlayer 151).

That is, the Mg concentration of the p-type contact layer 150 is low onthe light emitting section 140 side and high on the surface side(opposite side from the light emitting section 140). This can cancel thepiezoelectric field in the p-type confinement layer 151 hampering holeinjection, and improve the carrier confinement effect in conjunctionwith reducing the operating voltage.

More specifically, the efficiency was increased when the Mgconcentration on the p-type confinement layer 151 side of the p-typecontact layer 150 was 1×10¹⁹ cm⁻³ and the Mg concentration on thesurface side (opposite side from the p-type confinement layer) was5-9×10¹⁹ cm⁻³.

When the Mg concentration near the surface of the p-type contact layer150 is 1×10²⁰ cm⁻³ or more, diffusion into the light emitting section140 occurs, degrading the efficiency and reliability. At 5×10¹⁹ cm⁻³ orless, the operating voltage increases.

The p-type confinement layer 151 is preferably made of Al_(x)Ga_(1-x)N(0.2≦x≦0.32). That is, devices with x being 0.2-0.32 in the abovecomposition formula exhibited good characteristics, whereas theefficiency was low for devices fabricated with x being 0.35. Hence, thep-type confinement layer 151 is preferably made of Al_(x)Ga_(1-x)N(0.2≦x≦0.32).

Second Embodiment

FIG. 4 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to asecond embodiment of the invention.

As shown in FIG. 4, the semiconductor light emitting device 20 accordingto the second embodiment of the invention has a structure formed bystacking, on a substrate 110 having a sapphire c-plane surface, a firstbuffer layer 121 made of highly carbon-doped AlN, a second buffer layer122 made of high-purity AlN, a lattice-relaxed layer 123 made ofnon-doped GaN, an n-type contact layer 130 made of Si-doped n-type GaN,an n-type confinement layer 131 made of Si-doped n-type AlGaN, a lightemitting section 140, a p-type confinement layer 151 made of Mg-dopedp-type AlGaN, and a p-type contact layer 150 made of Mg-doped p-typeGaN.

The first buffer layer 121, the second buffer layer 122, thelattice-relaxed layer 123, the n-type contact layer 130, the n-typeconfinement layer 131, the p-type confinement layer 151, and the p-typecontact layer 150 can be based on the same material and configuration asthose described in the first embodiment.

In the semiconductor light emitting device 20 according to thisembodiment, the light emitting section 140 has a multiple quantum wellstructure in which a barrier layer 141 made of Si-doped n-type AlGaInNand a well layer 142 made of GaInN are alternately stacked six times.

Here, the aforementioned AlGaInN is Al_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y,x+y≦1).

The well layer 142 can be made of not only GaInN but also AlGaInN.

Furthermore, in the light emitting section 140 of the semiconductorlight emitting device 20 according to this embodiment, the barrier layer141 sandwiched between the well layers 142, that is, the inner barrierlayer 141 c, has a distributed Si concentration. That is, the innerbarrier layer 141 c has a stacked structure including a third portion143 c with low Si concentration and a fourth portion 143 d with high Siconcentration.

More specifically, in the light emitting section 140, each barrier layer141 sandwiched between the well layers 142, that is, each inner barrierlayer 141 c, includes a layered third portion 143 c provided on then-type confinement layer (first layer) 131 side and made ofSi-containing Al_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), and a layeredfourth portion 143 d provided on the p-type confinement layer (secondlayer) 151 side and made of Al_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1)which contains Si at a concentration higher than the Si concentration inthe third portion 143 c.

The third portion 143 c with low Si concentration can be illustrativelymade of Al_(0.08)Ga_(0.9)In_(0.02)N with, for instance, a Siconcentration of 1×10¹⁶ cm⁻³ and a thickness of 4.5 nm.

The fourth portion 143 d with high Si concentration can beillustratively made of Al_(0.08)Ga_(0.9)In_(0.02)N with, for instance, aSi concentration of 2×10¹⁹ cm⁻³ and a thickness of 9 nm.

However, the Si concentration in the third portion 143 c only needs tobe relatively lower than the Si concentration in the fourth portion 143d. That is, each inner barrier layer 141 c only needs to have a Siconcentration distribution which decreases from the p-type confinementlayer 151 side to the n-type confinement layer 131 side.

Here, the Si concentration in the p-side barrier layer 141 a can be setlower than the Si concentration in the n-side barrier layer 141 b.

For instance, the p-side barrier layer 141 a can be made ofAl_(0.08)Ga_(0.9)In_(0.02)N with low Si concentration with, forinstance, a Si concentration of 1×10¹⁶ cm⁻³ and a thickness of 9 nm. TheSi concentration in the p-side barrier layer 141 a may be comparable toor different from the Si concentration in the third portion 143 c. Thethickness may also be equal or different. Furthermore, the n-sidebarrier layer 141 b can be made of Al_(0.08)Ga_(0.9)In_(0.02)N with, forinstance, a Si concentration of 2×10¹⁹ cm⁻³ and a thickness of 9 nm.

The well layer 142 of the light emitting section 140 can beillustratively made of GaInN with a thickness of 4.5 nm, and the peakwavelength of light emitted from the light emitting section 140 can bein the near-ultraviolet region, 370-400 nm.

Each third portion 143 c with low Si concentration is located nearer then-type confinement layer 131 than each fourth portion 143 d with high Siconcentration.

Four well layers 142 are provided, and the third portion 143 c with lowSi concentration and the fourth portion 143 d with high Si concentrationare sandwiched between the well layers 142. That is, the third portion143 c and the highly Si-doped fourth portion 143 d constituting theinner barrier layer 141 c are provided between each pair of the fourwell layers 142.

Thus, in the semiconductor light emitting device 20 according to thisembodiment, the inner barrier layer 141 c is formed from the combinationof the third portion 143 c with low Si concentration and the fourthportion 143 d with high Si concentration.

In this example, the third portion 143 c with low Si concentration isimplemented by a non-doped layer so that the Si concentrationdistribution is changed between the third portion 143 c with low Siconcentration and the fourth portion 143 d with high Si concentration.This concentration distribution is not limited to the concentrationdifference in two steps, but the concentration may be variedcontinuously and gradually. The variation may be linear or nonlinear.

Next, a description is given of the relationship between the Siconcentration distribution in the barrier layer 141 configured asdescribed above and the piezoelectric field applied to the well layer142.

A piezoelectric field is applied to the well layer 142. Hence, at theinterface between the well layer 142 and the barrier layer 141 incontact therewith on the p-type confinement layer 151 side, positivecharges penetrate from the well layer 142 to the barrier layer 141. Onthe other hand, at the interface between the well layer 142 and thebarrier layer 141 in contact therewith on the n-type confinement layer131 side, negative charges penetrate from the well layer 142 to thebarrier layer 141.

Because many electrons exist on the p-type confinement layer 151 side ofthe well layer 142, there is no need to supply electrons from thebarrier layer 141. Hence, the barrier layer 141 in contact with thisinterface can be at a low Si concentration. Thus, a third portion 143 cwith low Si concentration is located in the barrier layer 141 so as tobe in contact with the well layer 142 on the p-type confinement layer151 side.

On the other hand, because few electrons exist on the n-type confinementlayer 131 side of the well layer 142, it is necessary to supplyelectrons from the barrier layer 141. Hence, the barrier layer 141 incontact with this interface needs to have a high Si concentration. Thus,a fourth portion 143 d with high Si concentration is located in thebarrier layer so as to be in contact with the well layer 142 on then-type confinement layer 131 side.

That is, the third portion 143 c with low Si concentration is located onthe p-type confinement layer 151 side of the well layer 142, and thefourth portion 143 d with high Si concentration is located on the n-typeconfinement layer 131 side of the well layer 142. In other words, ineach inner barrier layer 141 c sandwiched between the well layers 142,the Si concentration decreases from the p-type confinement layer 151side to the n-type confinement layer 131 side.

By varying the Si concentration in the inner barrier layer 141 c asdescribed above, the emission efficiency can be increased. In addition,the half-width of the emission spectrum can be narrowed.

More specifically, at the interface between the well layer 142 and thefourth portion 143 d in contact therewith on the n-type confinementlayer 131 side, a large quantity of electrons flow from the fourthportion 143 d with high Si concentration into the well layer 142,leaving a large quantity of positively charged Si in the fourth portion143 d. The distribution of electron concentration and Si concentrationat this interface serves to cancel the piezoelectric field, and,consequently the piezoelectric field is weakened. As the piezoelectricfield is weakened, the energy band of MQW warped by the piezoelectricfield is flattened, and accordingly the emission efficiency isincreased. Furthermore, the half-width of the emission spectrum isnarrowed.

As described above, in the semiconductor light emitting device 20according to this embodiment, the Si concentration is varied inside theinner barrier layer 141 c to control the electric field applied to thelight emitting section 140, thereby increasing the emission efficiency.Thus, this embodiment can provide a semiconductor light emitting deviceachieving near-ultraviolet emission with high efficiency.

Furthermore, in the semiconductor light emitting device 20 according tothis embodiment, the Si concentration in the p-side barrier layer 141 acan be set lower than the Si concentration in the n-side barrier layer141 b to improve reliability and reduce the driving voltage of thesemiconductor light emitting device 20.

That is, the decreased Si concentration in the p-side barrier layer 141a reduces electrons which overflow from the well layer 142 nearest thep-type confinement layer 151 to the p-type confinement layer 151 side.Thus, the reliability of the semiconductor light emitting device isimproved.

Furthermore, the decreased Si concentration in the p-side barrier layer141 a lowers the energy level of the p-side barrier layer 141 a andtends to exclude holes, which is effective in voltage reduction of thesemiconductor light emitting device 20.

As described above, in the semiconductor light emitting device 20according to this embodiment, the decreased Si concentration in thep-side barrier layer 141 a serves to increase the emission efficiency.Thus, this embodiment can provide a semiconductor light emitting deviceachieving near-ultraviolet emission with high efficiency, highreliability, and low driving voltage.

In the above example, the Si concentration in the p-side barrier layer141 a is uniform. However, like the semiconductor light emitting device10 according to the first embodiment, the p-side barrier layer 141 a caninclude a first portion 143 a with high Si concentration and a secondportion 143 b with low Si concentration. In this case, the distributionof Si concentration can be varied stepwise or continuously as long asthe Si concentration in the p-side barrier layer 141 a is set lower thanin the n-side barrier layer 141 b. Furthermore, the Si concentration inthe p-side barrier layer 141 a can be set lower than the Siconcentration in the inner barrier layer 141 c, that is, the Siconcentration in the combination of the third portion 143 c and thefourth portion 143 d.

Next, a technique for growing the barrier layer 141 is described. Aquaternary mixed crystal AlGaInN layer having good crystal quality isdifficult to grow. Furthermore, the crystal is prone to degradation whenhighly doped with Si. By studying the LED device structure andoptimizing the growth condition, we have succeeded in increasing the Incomposition ratio of the barrier layer 141 made of AlGaInN withoutdegradation in crystal quality.

For instance, in the semiconductor light emitting device 10 according tothe first embodiment, the barrier layer 141 has a large thickness, andhence the In composition has a limit of approximately 1%. However, inthe structure of the semiconductor light emitting device 20 according tothis embodiment, the thickness of the barrier layer (fourth portion 143d) highly doped with Si in the inner barrier layer 141 c can bedecreased. Hence, the crystal is not degraded even if it contains 2% In,and intense light emission is achieved.

A high In composition ratio improves the steepness of the interface withthe well layer 142 made of GaInN, enhances the crystallinity of MQW, andconsequently allows the fourth portion 143 d made of AlGaInN to behighly doped with Si.

Furthermore, the decreased thickness of the fourth portion 143 d withhigh Si concentration allows Si doping at a higher concentration.

In the semiconductor light emitting device 20 according to thisembodiment, forming a fourth portion 143 d with very high Siconcentration is important, and serves to significantly increase theemission efficiency.

Third Embodiment

FIG. 5 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to athird embodiment of the invention.

As shown in FIG. 5, in the semiconductor light emitting device 30according to the third embodiment of the invention, a projections anddepressions 150 a is provided at the interface of the p-type contactlayer 150 with the p-side electrode 160. The rest is the same as thesemiconductor light emitting device 10 according to the firstembodiment, and hence the description thereof is omitted.

The projections and depressions 150 a improves contact between thep-type contact layer 150 and the p-side electrode 160, and also has theeffect of diffusing emitted light and radiating it outside.

In the semiconductor light emitting device 30 having such structure,this embodiment can provide a semiconductor light emitting deviceachieving near-ultraviolet emission with higher efficiency.

The aforementioned projections and depressions 150 a can be provided inthe second semiconductor light emitting device 20. More specifically,the inner barrier layer 141 c of the light emitting section 140 can be acombined barrier of a third portion 143 c with low Si concentration anda fourth portion 143 d with high Si concentration. Here, furthermore, asdescribed in the first embodiment, the p-side barrier layer 141 a caninclude a first portion 143 a with high Si concentration and a secondportion 143 b with low Si concentration. Also in these cases, a similareffect of the projections and depressions 150 a is achieved.

Fourth Embodiment

FIG. 6 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to afourth embodiment of the invention.

As shown in FIG. 6, in the semiconductor light emitting device 40according to the fourth embodiment of the invention, on an n-type GaNwafer 133, the n-type confinement layer 131, the light emitting section140, the p-type confinement layer 151, and the p-type contact layer 150described in the first embodiment are sequentially provided. A p-sideelectrode 160 is provided on the p-type contact layer 150. Furthermore,projections and depressions 133 b are provided at the backside 133 a ofthe n-type GaN wafer 133, and an n-side electrode 170 is provided incontact with the projections and depressions 133 b at the backside 133 aof the n-type GaN wafer 133. The rest is the same as the semiconductorlight emitting device 10 according to the first embodiment, and hencethe description thereof is omitted.

The semiconductor light emitting device 40 according to this embodimentdoes not use the substrate 110 made of sapphire described in the firstembodiment, but uses an n-type GaN wafer 133 having crystal quality withlow dislocation density.

In the semiconductor light emitting device 40 according to thisembodiment, use of the n-type GaN wafer 133 allows a current to flow inthe thickness direction of the n-type GaN wafer 133, which has theeffect of decreasing the operating voltage.

In the semiconductor light emitting device 40 having such structure,like the first embodiment, this embodiment can also provide asemiconductor light emitting device achieving near-ultraviolet emissionwith high efficiency and low operating voltage.

The n-type GaN wafer 133 described in this embodiment is also applicableto the semiconductor light emitting device 20 according to the secondembodiment. More specifically, the inner barrier layer 141 c of thelight emitting section 140 can be a combined barrier of a third portion143 c with low Si concentration and a fourth portion 143 d with high Siconcentration. Here, furthermore, as described in the first embodiment,the p-side barrier layer 141 a can include a first portion 143 a withhigh Si concentration and a second portion 143 b with low Siconcentration. Also in these cases, a similar effect of the n-type GaNwafer 133 is achieved.

Fifth Embodiment

FIG. 7 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to afifth embodiment of the invention.

As shown in FIG. 7, in the semiconductor light emitting device 50according to the fifth embodiment of the invention, on an n-type GaNwafer 133, the n-type confinement layer 131, the light emitting section140, the p-type confinement layer 151, and the p-type contact layer 150described in the first embodiment are sequentially provided. Pprojections and depressions 150 a are provided at the upper surface ofthe p-type contact layer 150, and a p-side electrode 160 is providedthereon. Furthermore, projections and depressions 133 b are provided atthe backside 133 a of the n-type GaN wafer 133, and an n-side electrode170 is provided in contact with the projections and depressions 133 b atthe backside 133 a of the n-type GaN wafer 133. The rest is the same asthe semiconductor light emitting device 10 according to the firstembodiment, and hence the description thereof is omitted.

The semiconductor light emitting device 50 according to this embodimentdoes not use the substrate 110 made of sapphire described in the firstembodiment, but uses an n-type GaN wafer 133 having crystal quality withlow dislocation density.

In the semiconductor light emitting device 50 according to thisembodiment, use of the n-type GaN wafer 133 allows a current to flow inthe thickness direction of the n-type GaN wafer 133, which has theeffect of decreasing the operating voltage.

In the semiconductor light emitting device 50 having such structure,like the first embodiment, this embodiment can also provide asemiconductor light emitting device achieving near-ultraviolet emissionwith high efficiency and low operating voltage.

Furthermore, the efficiency can be further increased by providingprojections and depressions 133 b at the backside of the n-type GaNwafer 133 in conjunction with providing projections and depressions 150a in the p-type contact layer 150.

The n-type GaN wafer 133 and the projections and depressions 150 adescribed in this embodiment are also applicable to the semiconductorlight emitting device 20 according to the second embodiment.

Sixth Embodiment

FIG. 8 is a schematic cross-sectional view illustrating theconfiguration of a wafer according to a sixth embodiment of theinvention.

As shown in FIG. 8, the wafer 60 according to the sixth embodiment ofthe invention comprises: a first layer 131 made of at least one ofn-type GaN and n-type AlGaN; a second layer made of Mg-containing p-typeAlGaN; and a light emitting section 140 provided between the first layer131 and the second layer 151 and including a plurality of barrier layers141 made of Si-containing Al_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), anda well layer 142 provided between each pair of the plurality of barrierlayers 141 and made of GaInN or AlGaInN, one of the barrier layers 141nearest the second layer including a first portion 143 a made ofSi-containing Al_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), and a secondportion 143 b provided between the first portion 143 a and the secondlayer 151 and made of Al_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1) whichcontains Si at a concentration lower than Si concentration in the firstportion 143 a and lower than Si concentration in the barrier layersexcept the one barrier layer nearest the second layer 151. That is, theplurality of barrier layers 141 have a nearest barrier layer and a farbarrier layer. The nearest barrier layer is nearest to the second layeramong the plurality of barrier layers. The nearest barrier layer has thefirst portion 143 a and the second portion 143 b.

In the wafer 60 having such structure, this embodiment can provide awafer achieving near-ultraviolet emission with high efficiency like thefirst embodiment.

As described with reference to the first embodiment, in the wafer 60,the Si concentration in the first portion 143 a can be not more than theSi concentration in the barrier layers 141 (n-side barrier layer 141 band inner barrier layers 141 c) except the barrier layer 141 nearest thesecond layer 151. The first portion 143 a is in contact with the welllayer 142.

The thickness of the second portion 143 b is preferably 3 nm or more and9 nm or less.

The Si concentration in the second portion 143 b is preferably 1×10¹⁶cm⁻³ or more and 9.0×10¹⁸ cm⁻³ or less.

The Si concentration in the barrier layers 141 (n-side barrier layer 141b and inner barrier layers 141 c) except the barrier layer 141 nearestthe second layer 151 is preferably 1.1×10¹⁹ cm⁻³ or more and 3×10¹⁹ cm⁻³or less.

The Si concentration in the first portion 143 a is preferably 1.1×10¹⁹cm⁻³ or more and 3×10¹⁹ cm⁻³ or less.

Seventh Embodiment

FIG. 9 is a schematic cross-sectional view illustrating theconfiguration of a wafer according to a seventh embodiment of theinvention.

As shown in FIG. 9, the wafer 70 according to the seventh embodiment ofthe invention comprises: a first layer 131 made of at least one ofn-type GaN and n-type AlGaN; a second layer 151 made of Mg-containingp-type AlGaN; and a light emitting section 140 provided between thefirst layer 131 and the second layer 151 and including a plurality ofbarrier layers 141 made of Si-containing Al_(x)Ga_(1-x-y)In_(y)N (0≦x,0≦y, x+y≦1), and a well layer 142 provided between each pair of theplurality of barrier layers 141 and made of GaInN or AlGaInN, each ofthe barrier layers 141 sandwiched between the well layers 142 includinga third portion 143 c provided on the first layer 131 side and made ofSi-containing Al_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), and a fourthportion 143 d provided on the second layer 151 side and made ofAl_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1) which contains Si at aconcentration higher than Si concentration in the third portion 143 c.

In the wafer 70 having such structure, this embodiment can provide awafer achieving near-ultraviolet emission with high efficiency like thesecond embodiment.

As described with reference to the second embodiment, in the wafer 70,the Si concentration in the barrier layer 141 (p-side barrier layer 141a) nearest the second layer 151 can be set lower than the Siconcentration in the barrier layer 141 (n-side barrier layer 141 b)nearest the first layer 131.

The barrier layer 141 (p-side barrier layer 141 a) nearest the secondlayer 151 can include a first portion 143 a made of Si-containingAl_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), and a second portion 143 bprovided between the first portion 143 a and the second layer 151 andmade of Al_(x)Ga_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1) which contains Si at aconcentration lower than Si concentration in the first portion 143 a.

The Si concentration in the fourth portion 143 d is preferably 1.1×10¹⁹cm⁻³ or more and 3×10¹⁹ cm⁻³ or less.

The Si concentration in the barrier layer 141 (n-side barrier layer 141b) nearest the first layer 131 is preferably 1.1×10¹⁹ cm⁻³ or more and3×10¹⁹ cm⁻³ or less.

As described with reference to the first and second embodiment, in thewafer 60 and the wafer 70, the second layer 151 can be made ofAl_(x)Ga_(1-x)N (0.2≦x≦0.32).

The Mg concentration on the light emitting section 140 side of thesecond layer 151 can be set higher than the Mg concentration on theopposite side of the second layer 151 from the light emitting section140.

The Mg concentration of the second layer 151 can be set to 8×10¹⁸ cm⁻³or more and 3×10¹⁹ cm⁻³ or less.

The wafer 60 and the wafer 70 can further comprise a third layer (p-typecontact layer) 150 provided on the opposite side of the second layer 151from the light emitting section 140 and made of at least one of p-typeGaN and p-type AlGaN, wherein the Mg concentration on the first layer131 side of the third layer 150 can be set lower than the Mgconcentration on the opposite side of the third layer 150 from the firstlayer 131.

In the wafer 60 and the wafer 70, the peak wavelength of light emittedfrom the light emitting section 140 can be set in the range of 370-400nm.

The light emitting section 140 can be provided on a GaN layer grownabove a substrate 110 having a sapphire c-plane via a single crystalbuffer layer having a composition range of Al_(x)Ga_(1-x)N (0.8≦x≦1).

The single crystal buffer layer can include a second buffer layer 122provided above the substrate 110, and a first buffer layer providedbetween the second buffer layer 122 and the substrate 110, having ahigher carbon concentration than the second buffer layer 122, and havinga thickness of 3 nm or more and 20 nm or less.

The wafer 60 and the wafer 70 according to the embodiments of theinvention can be provided with the projections and depressions 150 a,133 b described in the third, fourth, and fifth embodiment.

The embodiments of the invention have been described with reference toexamples. However, the invention is not limited to these examples. Forinstance, various specific configurations of the components constitutingthe semiconductor light emitting device and the wafer are encompassedwithin the scope of the invention as long as those skilled in the artcan similarly practice the invention and achieve similar effects bysuitably selecting such configurations from conventionally known ones.

Furthermore, any two or more components of the examples can be combinedwith each other as long as technically feasible, and such combinationsare also encompassed within the scope of the invention as long as theyfall within the spirit of the invention.

Furthermore, those skilled in the art can suitably modify and implementthe semiconductor light emitting device and the wafer described above inthe embodiments of the invention, and any semiconductor light emittingdevice and wafer thus modified are also encompassed within the scope ofthe invention as long as they fall within the spirit of the invention.

Furthermore, those skilled in the art can conceive various modificationsand variations within the spirit of the invention, and it is understoodthat such modifications and variations are also encompassed within thescope of the invention.

1. A semiconductor light emitting device comprising: a first layer madeof at least one of n-type GaN and n-type AlGaN; a second layer made ofMg-containing p-type AlGaN; and a light emitting section providedbetween the first layer and the second layer and including a pluralityof barrier layers made of Si-containing AlxGa_(1-x-y)In_(y)N (0≦x, 0≦y,x+y≦1), and a well layer provided between each pair of the plurality ofbarrier layers and made of GaInN or AlGaInN, each of the barrier layerssandwiched between the well layers including a third portion provided onthe first layer side and made of Si-containing AlxGa_(1-x-y)In_(y)N(0≦x, 0≦y, x+y≦1), and a fourth portion provided on the second layerside and made of AlxGa_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), a Siconcentration in the fourth portion being higher than Si concentrationin the third portion, wherein the barrier layer nearest to the secondlayer includes: a first portion made of Si-containingAlxGa_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1); and a second portion providedbetween the first portion and the second layer and made ofAlxGa_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), a Si concentration in the secondportion being lower than a Si concentration in the first portion.
 2. Thedevice according to claim 1, wherein the Si concentration in the barrierlayer nearest the second layer is lower than the Si concentration in thebarrier layer nearest the first layer.
 3. The device according to claim1, wherein the Si concentration in the fourth portion is 1×10¹⁹ cm⁻³ ormore and 3.0×10¹⁹ cm⁻³ or less.
 4. The device according to claim 1,wherein a Si concentration in a the barrier layer nearest to the firstlayer is 1.1×10¹⁹ cm⁻³ or more and 3×10¹⁹ cm⁻³ or less.
 5. The deviceaccording to claim 1, wherein the second layer is made ofAl_(x)Ga_(1-x)N (0.2≦x≦0.32).
 6. The device according to claim 1,wherein a Mg concentration on the light emitting section side of thesecond layer is higher than a Mg concentration on the opposite side ofthe second layer from the light emitting section.
 7. The deviceaccording to claim 1, wherein the Mg concentration in the second layeris 8×10¹⁸ cm⁻³ or more and 3×10¹⁹ cm⁻³ or less.
 8. The device accordingto claim 1, further comprising a third layer provided on the oppositeside of the second layer from the light emitting section and made of atleast one of p-type GaN and p-type AlGaN, a Mg concentration on thefirst layer side of the third layer being lower than a Mg concentrationon the opposite side of the third layer from the first layer.
 9. Thedevice according to claim 1, wherein light emitted from the lightemitting section has a peak wavelength in a range of 370 nanometers ormore and 400 nanometers or less.
 10. The device according to claim 1,wherein the light emitting section is provided on a GaN layer grownabove a substrate having a sapphire c-plane via a single crystal bufferlayer having a composition range of Al_(x)Ga_(1-x)N (0.8≦x≦1).
 11. Thedevice according to claim 10, wherein the single crystal buffer layerincludes a first buffer layer and a second buffer layer, the secondbuffer layer being provided above the substrate, and the first bufferlayer being provided between the second buffer layer and the substrate,the first buffer layer having a higher carbon concentration than thesecond buffer layer, and having a thickness of 3 nanometers or more and20 nanometers or less.
 12. A wafer comprising: a first layer made of atleast one of n-type GaN and n-type AlGaN; a second layer made ofMg-containing p-type AlGaN; and a light emitting section providedbetween the first layer and the second layer and including a pluralityof barrier layers made of Si-containing AlxGa_(1-x-y)In_(y)N (0≦x, 0≦y,x+y≦1), and a well layer provided between each pair of the plurality ofbarrier layers and made of GaInN or AlGaInN, each of the barrier layerssandwiched between the well layers including a third portion provided onthe first layer side and made of Si-containing AlxGa_(1-x-y)In_(y)N(0≦x, 0≦y, x+y≦1), and a fourth portion provided on the second layerside and made of AlxGa_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), a Siconcentration in the fourth portion being higher than Si concentrationin the third portion, wherein the barrier layer nearest to the secondlayer includes: a first portion made of Si-containingAlxGa_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1); and a second portion providedbetween the first portion and the second layer and made ofAlxGa_(1-x-y)In_(y)N (0≦x, 0≦y, x+y≦1), a Si concentration in the secondportion being lower than a Si concentration in the first portion. 13.The wafer according to claim 12, wherein the Si concentration in thebarrier layer nearest the second layer is lower than the Siconcentration in the barrier layer nearest the first layer.
 14. Thewafer according to claim 12, wherein the Si concentration in the fourthportion is 1×10¹⁹ cm⁻³ or more and 3.0×10¹⁹ cm⁻³ or less.
 15. The waferaccording to claim 12, wherein a Si concentration in a the barrier layernearest to the first layer is 1.1×10¹⁹ cm⁻³ or more and 3×10¹⁹ cm⁻³ orless.
 16. The wafer according to claim 12, wherein the second layer ismade of Al_(x)Ga_(1-x)N (0.2≦x≦0.32).
 17. The wafer according to claim12, wherein a Mg concentration on the light emitting section side of thesecond layer is higher than a Mg concentration on the opposite side ofthe second layer from the light emitting section.
 18. The waferaccording to claim 12, wherein the Mg concentration in the second layeris 8×10¹⁸ cm⁻³ or more and 3×10¹⁹ cm⁻³ or less.
 19. The wafer accordingto claim 12, further comprising a third layer provided on the oppositeside of the second layer from the light emitting section and made of atleast one of p-type GaN and p-type AlGaN, a Mg concentration on thefirst layer side of the third layer being lower than a Mg concentrationon the opposite side of the third layer from the first layer.
 20. Thewafer according to claim 12, wherein light emitted from the lightemitting section has a peak wavelength in a range of 370 nanometers ormore and 400 nanometers or less.